海南岛陆缘扩张带形成及新生代岩石圈动力学机制:来自幔源包体的地球化学证据

摘要

海南岛陆缘扩张带蓬莱地区新生代玄武岩中捕获大量尖晶石二辉橄榄岩和方辉橄榄岩幔源包体.激光剥蚀等离子体质谱(LA-ICP-MS)分析结果表明,蓬莱地幔橄榄岩含有三种不同地球化学特征的单斜辉石(Cpx):(1)a类单斜辉石Mg#=92.3～93.4,来自富集Cpx的二辉橄榄岩,具有极低的LREE和不相容元素含量,HREE平坦,Th、U、La、Sr正异常,经历了7％～10％的尖晶石相部分熔融,仅受到极低程度强不相容元素(Th、U、La、Sr)初期富集交代作用;(2)b类单斜辉石Mg#=89.9～90.3,来自较富集Cpx的二辉橄榄岩,具有中等的LREE和LILE含量,HREE平坦,微量元素蛛网图上显示Th、U正异常,Rb、Ba、Nb、Ta、Sr、Ti负异常,经历4％～5％的尖晶石相部分熔融,可能受到了含LREE和Th、U等不相容元素的硅酸盐熔体交代;(3)c类单斜辉石Mg#=91.4～92.8,来自贫Cpx的二辉橄榄岩和方辉橄榄岩,具有富集的LREE和LILE含量,HREE弱分异,微量元素蛛网图上显示Th、U正异常及强烈的Nb、Ta、Ti负异常,经历了8％～20％的尖晶石相部分熔融,其交代熔体可能是来自源区有石榴子石残留的碳酸盐熔体.全岩主、微量元素及模拟计算结果表明,这些幔源包体的主量元素主要受部分熔融程度影响,并且方辉橄榄岩经历的部分熔融程度大于二辉橄榄岩.地幔橄榄岩的Sr-Nd同位素组成表明该区具有MORB-OIB型亏损地幔特征.此外,蓬莱部分地幔橄榄岩包体显示正斜率的HREE分异特征((Gd/Yb)N=0.4～0.7),暗示该区地幔经历了源自石榴子石稳定区的变压熔融,总体熔融程度为18％以上,指示了较高的地幔潜能温度.综合前人对海南岛新生代玄武岩最新研究成果,我们认为海南地幔柱可能为该区软流圈地幔置换古老岩石圈地幔提供了热源,导致了区域岩石圈地幔的破坏,从而引起包括地幔柱本身、软流圈和富集岩石圈的熔融.岩石圈地慢性质的改变和不均一性可能是海南岛陆缘扩张带新生代岩石圈减薄的主要动力学机制.%The mantle xenoliths (spinel lherzolites and harzburgites) that were brought to the surface by the Penglai Cenozoic alkali basalts in the Leiqiong active area of East Asia continental margin extensional belt have been studied to constrain the nature of the upper mantle and the mantle processes.Analytical results obtained by Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and other techniques suggest that the variation of the major elements of mantle xenoliths is primarily controlled by the partial melting degrees,and the harzburgites experienced higher degrees of partial melting than lherzolites.The whole-rock Sr-Nd isotopes indicate a MORB-OIB type depleted mantle beneath Hainan Island.Three types of clinopyroxene with distinctive geochemistry characteristics are recognized among these xenoliths.(1) The type-A clinopyroxenes from the Cpx-rich lherzolites,with Mg#=92.3-93.4,have experienced 7％-10％ partial melting.The contents of their LREE and incompatible elements are extremely low,with flat heavy REE (HREE) patterns and positive spikes of Th,U,La and Sr,indicating incipient enrichment of highly incompatible elements.(2) The type-B clinopyroxenes from the lherzolites containing relatively more Cpx,with Mg#=89.9-90.3,experienced only 4％-5％ partial melting.Their LREE and incompatible elements concentrations are comparatively high,with flat HREE patterns,positive anomalies of Th and U,and negative anomalies of Rb,Ba,Nb,Ta,Sr and Ti,suggesting silicate melts percolation with enriched LREE and LILE contents.(3) The type-C clinopyroxenes from the Cpx-poor lherzolites and harburgites,with Mg#=91.4-92.8,experienced 8％-20％ partial melting.Their LREE and incompatible elements contents show a steep enrichment,coupled with fractionated HREE patterns,Th and U enrichment,whereas Nb,Ta and Ti are strongly depleted.Their characteristics indicate that the percolating melts could be the carbonate melts from deeper mantle with garnet residues.Besides,parts of the Penglai mantle peridotites show fractionated HREE characteristics ((Gd/Yb)N=0.4-0.7),suggesting that the mantle beneath the Hainan Island may have experienced polybaric melting initially from garnet stability field with high potential mantle temperature.The modelling suggests these peridotites experienced about 18％ polybaric melting with 5％ in the garnet stability field and the other 13％ in the spinel stability field.Integrated with the previous studies of the Cenozoic basalts in the Leiqiong active region,we conclude that the Hainan mantle plume could be the origin of high potential mantle temperature and it may provide the heat source of partial replacement of Archean-Proterozoic lithosphere by asthenosphere,which rose adiabatically subsequent to lithospheric thinning during the Cenozoic Era.The modification and heterogeneity of the lithosphere caused by the mantle plume and the subsequently melting of the mantle plume,asthenosphere and enriched lithosphere are probably the main dynamic mechanism triggering the lithospheric thinning of the Leiqiong active area in East Asia continental margin extension belt.